A photodiode normally includes an n-type semiconductor region and a p-type semiconductor region formed by selectively diffusing an impurity (e.g. boron) into the n-type semiconductor region. The p-type semiconductor region and n-type semiconductor region form a p-n junction. When light of an appropriate intensity arrives at the photodiode, electron-hole pairs are generated throughout the whole body of the photodiode, i.e. in the depletion layer near the junction of the p-type and n-type semiconductor regions, in the p-type semiconductor region, and in the n-type semiconductor region. Normally, in the depletion layer, electrons and holes are accelerated toward the n-type semiconductor region and the p-type semiconductor region, respectively, due to the effect of an electric field, Among the electron-hole pairs generated in the n-type semiconductor region, the electrons remain within the n-type semiconductor region along with the electrons transferred from the n-type semiconductor region, while the holes diffuse within the n-type semiconductor region to the depletion layer. Upon reaching the depletion layer, the holes are accelerated by the electric field and gathered into the p-type semiconductor region. In this manner, holes and electrons are collected in the p-type and n-type semiconductor regions, respectively, and flow through an externally connected load as a photocurrent.
Even when the conductivity types of the semiconductors are opposite to the previously described ones, the construction and operation of the device are basically the same.
In commonly used photodiodes, the p-type semiconductor region formed by the selective diffusion almost entirely covers the light-receiving area which receives incident light, so as to allow the light to reach the entire junction between the p-type and n-type semiconductor regions. FIG. 15A is a schematic sectional view of a typical photodiode, and FIG. 15B is its top-side plan view. In the present example, the base body 11 itself serves as the n-type semiconductor region, with the p-type semiconductor region 12 formed by selective diffusion across almost the same range as the light-receiving area 10 on the surface of the base body 11. The contact part 13, which consists of a conductor formed in contact with the base body 11, is the cathode terminal (C), while the contact part 14, which consists of a conductor formed in contact with the p-type semiconductor region 12, is the anode terminal (A).
In order to achieve a high level of photo-detection sensitivity in such a photodiode, the light-receiving area 10 should preferably have a large area. If the light-receiving area 10 is enlarged, the p-type semiconductor region 12 also needs to be enlarged due to the previously described reason. However, increasing the area of the p-type semiconductor region 12 causes an increase in the junction capacitance, which in turn increases the level of noise in such elements as the amplifier connected for the conversion of the photocurrent produced by the photodiode into voltage. Consequently, the SN ratio of the photo-detection signal becomes lower, which makes it necessary to reduce the frequency bandwidth of the amplifier or decrease its gain.
In other words, to reduce a high-frequency noise in a photodiode, the junction capacitance needs to be lowered (for example, see Patent Literature 1). However, if the area of the p-type semiconductor region formed by selective diffusion is decreased in order to lower the junction capacitance, the photo-detection sensitivity may possibly be lowered.
In order to lower the junction capacitance of the p-n junction, in a conventional photodiode described in Patent Literature 2, a structure is adopted in which a plurality of island-like p-type diffusion layers are formed on the surface of an n-type substrate, and the same number of electrodes as the island-like diffusion layers are provided in a mutually connected form. The spacing of the island-like p-type diffusion layers is made to be equal to or smaller than the distance over which the minority carriers diffuse (“minority carrier diffusion length”). In Patent Literature 2, it is claimed that such a structure provides the photodiode with a larger light-receiving range which effectively works in both horizontal and vertical directions from each p-type diffusion layer, and the photocurrent corresponding to the incident light can also be obtained in the regions between the p-type diffusion layers (“no-diffusion-layer regions”), so that the photo-detection sensitivity barely deteriorates as compared to the case where the same diffusion layer is formed across the regions between the island-like p-type diffusion layers. It is also claimed that the junction capacitance becomes lower since the area of the p-n junction is decreased by an amount corresponding to the no-diffusion-layer regions.